The present invention relates generally to an apparatus and method for measuring power and power losses of light transmissions in fiber optic cables, and more specifically, the power and power losses resulting from the transmissions of known wavelengths of light.
Fiber optic cables have been finding increased applications in a variety of industries as a result of their lightweight composition and effectiveness in transferring information. The use of fiber optic cables has, for instance, become common in the telecommunications industry, where many of the bulkier and less efficient wire data lines are being replaced with fiber optic cables. To assure accuracy of the data transmitted by light signals, however, these fiber optic cables are tested by measuring the power losses associated with the light transmissions, and thus, determine the potential error in the data transmissions.
Light signals constituting data transmissions are transmitted through fiber optic cables using specific nominal wavelengths of light for the transmission. Because different wavelengths of light have varying attenuating characteristics, it is important to determine the attenuation of the fiber optic cable with respect to the specific wavelength of light which will be used for the particular data transmission.
In U.S. Pat. No. 4,234,253 to Higginbotham et al., an attenuation measuring system is described as having a transmitter and receiver which are operatively connected to opposite ends of a fiber optic cable under test. A reference signal comprised of a timing pulse is superimposed upon a square wave pulse. This modified square wave signal is then transmitted through the fiber optic cable where it is detected by a receiver attached to a second end of the cable. The receiver separates the timing pulse from the transmitted pulse for use in demodulating the transmitted signal and compares the demodulated square wave signal to a reference signal contained within the receiver. The use of a modified square wave pulse as a test signal, however, has the disadvantages of signal noise and limited bandwidth which are characteristic of DC signals.
U.S. Pat. No. 4,280,765 to Pophillat et al. describes a measuring system for measuring the transmission bandwidth of the fiber optic cable using a plurality of sinusoidal test signals as opposed to square wave pulses. Pophillat, therefore, avoids some of the disadvantages of Higginbotham. This system comprises a frequency generator for producing a first composite signal having discrete distribution of predetermined sinusoidal frequencies. A single laser is provided by which the first composite signal modulates the light signal for transmission through the optical fiber under test. An optical detector is positioned at a receiving end for converting the light signal into a second composite signal and directing the same to a spectrum analyzer. The analyzer provides a frequency spectrum display of the composite of the electrical sinusoidal signals which is compared with the spectrum of the first signal for determining the power attenuation of the first composite of signal transmissions. While the prior art has concentrated on techniques of measuring the response to the specific wavelength of light that is being transmitted, they have failed to address the problem of providing a tester which is capable of being adapted for the varying numbers of wavelengths which may be used. As more and more different sources of light become available, and the quality of light fibers increases, a single tester at a single wavelength will not be sufficient.
It is, therefore, an object of the present invention to provide a means for determining the power losses associated with any specific wavelengths of light transmissions.
Another object of the present invention is to provide a means for identifying the wavelength of light signals transmitted through a fiber optic cable under test and for determining the power losses associated with the respective light wavelengths.
Yet another object of the present invention is to provide a means for determining the power losses associated with specific wavelengths of light transmissions, using either a laser or an LED as a source for the light transmissions.
Still another object of the present invention is to provide a means for measuring the optical power transmitted through a fiber optic cable.
A further object of the present invention is to provide a microprocessor control tester for measuring the power attenuation at each specific wavelength of light transmissions in fiber optics.
These and other objects are attained by providing a transmitter for modulating the power intensities of known wavelengths of light with identifying or signature AC signals. The modulated light is transmitted through a length of fiber under test where the modulated light is then detected and the signature identified by a receiver. The transmitted AC signal is compared with a reference signal stored in a microprocessor for the specific signature to determine the power attenuation of the transmitted signal, thereby determining the transfer characteristics of the fiber optic cable as a function of light wavelengths. By using AC signals to modulate the light wavelength, the problem of noise and limited bandwidth associated with DC signal transmissions are avoided. Nevertheless, a specially preferred embodiment of the tester also includes the capabilities of also measuring the DC power of the transmitted signal.
A specially preferred embodiment of the tester is designed as a modular system, and thus, may be operated as either a single unit (local mode) or a plurality of separate units (remote mode). for fiber optic cables of relatively short length, a single tester unit having both a transmitter module and a receiver module is used, such that the single tester unit attaches to both ends of the cable under test. For cables of longer length, separate tester units having respective transmitting and receiving modules contained therein are provided, whereby the receiver unit includes independently calibrated reference signals for comparison with the received transmitted signal.
Preferred embodiments of the transmitting part of the tester in both single and separate tester operations may include a plurality of laser or light emitting diode (LED) source modules, each module designed to produce a predetermined known mean wavelength of light. Because fiber optic cables do not attenuate the power intensities of different wavelengths of light uniformly, a more accurate representation of the fiber optic cable transmission characteristics results with the use of a plurality of light sources. Each laser or LED source module has assigned to it a modulating AC signal of a specified frequency. After transmission of a nominal wavelength of light modulated on the assigned known frequency, the microprocessor controlled receiver module identifies the modulating frequency for determining which wavelength of light was transmitted, and thereby associate the power loss measurement with the transmitted wavelength of light.
In specially preferred embodiments with a laser light source, the transmitter includes a photodetector for providing input to two feedback circuits, which provide stabilization of the test signal amplitude and wavelength. These feedback loops stabilize the average power or DC level of the light source and maintains the percentage modulation constant. A third feedback which is utilized when the light source is either an LED or a laser, includes a temperature compensation circuit for maintaining the ambient temperature surrounding the light source at a selected temperature. This assures that the light source will have as consistent output as possible. Thus, the feedback circuits assure that during testing, the microprocessor controlled drive units of the respective light sources provide a proper modulation of the assigned wavelength of light.
A unique temperature compensating circuit for a light source is provided. This includes a heater/cooler in a four switch bridge which determines the direction of current through the heater/cooler as a function of temperature variations and, thus, the heating or cooling cycle of the heater/cooler.